Wireless sensors have a great deal of potential in numerous applications that are too expensive or impossible to monitor by wire. Wireless sensors are divided into battery-powered active devices containing a radio transmitter, semi-passive battery-assisted sensors using modulated backscattering technique, and fully passive sensors. The disadvantages of active sensors are their high cost and limitations due to the battery's lifetime. Existing passive and semi-passive wireless sensors are based either on silicon-based radio frequency identification (RFID) or surface acoustic wave (SAW) technology.
Advanced RFID technology enables low-cost passive and semi-passive tags (i.e., ID-sensors) with rewritable memory. The technology can be used to realize general sensor platform [1] (reference documents are specified in the list of references appended to this specification), and it has been used to realize for example temperature [2] and shock [3] sensors. The RFID technology has some drawbacks in sensor applications. The power rectifier that generates the required power for the IC limits both the highest operation frequency and the largest distance. Therefore, passive RFID tags are not feasible in applications where a high operation frequency or a large interrogation distance is required. High frequency enables the small antenna size and precise spatial localization of the tag.
SAW tags offer several advantages when used as a sensor: they are small, withstand harsh environment, enable a relatively long reading distance, are passive, and are inherently sensitive to some quantities without external sensor elements [4]. A disadvantage with SAW sensors is that they are expensive compared to silicon-based sensors due to higher prices of piezoelectric substrates. In addition, the highest operation frequency of a SAW sensor is limited by the line width of the metal pattern printed on the piezoelectric substrate and it is typically a few GHz.
The intermodulation distortion of MEMS devices have been studied in several articles. The intermodulation distortion in capacitive MEMS switches is approximated in [10] and [11]. These models assume small MEMS cantilever displacement and high bridge impedance and neglect the mechanical quality factor of the MEMS. More comprehensive analysis presented in [12] takes also fifth order non-linearities into account. Another approach based on Volterra-series is presented in [13]. An analytical model for intermodulation distortion due to contact heating in contacting MEMS switches is presented in [14]. In addition, the MEMS mixer-filters [15]-[18], also called mixlers, share somewhat similar operating principle to the proposed MEMS sensor.
Existing passive and semi-passive wireless sensors are based either on silicon-based radio frequency identification (RFID) or surface acoustic wave (SAW) technology. The RFID technology has some drawbacks in sensor applications. The power rectifier that generates the required power for the IC limits both the highest operation frequency and the largest distance. Therefore, passive RFID tags are not feasible in applications where a high operation frequency or a large interrogation distance is required. High frequency enables the small antenna size and precise spatial localization of the tag.
A disadvantage with SAW sensors is that they are expensive compared to silicon-based sensors due to higher prices of piezoelectric substrates. In addition, the highest operation frequency of a SAW sensor is limited by the line width of the metal pattern printed on the piezoelectric substrate and it is typically a few GHz.